Update header for accessibility

[Christopher Leonard]

see http://john.foliot.ca/aria-hidden/#h

aria-hidden="true" & tabindex="-1" on anchor element + "role="presentation" on both anchor and image elements (with <img alt="Tim Berners-Lee Takes the Stand to Keep the Web Free">)

Example below tested correctly in screen readers:

<a href="tbl.html" aria-hidden="true" role="presentation" tabindex="-1"> <img src="tim_th.jpg" class="thumb" alt="Tim Berners-Lee Takes the Stand to Keep the Web Free role="presentation""> </a>

[University of Delaware]

in production